Semiconductor technology employing both MOS and bipolar devices is extremely desirable because it allows for integrated circuits having the most attractive features of each technology. For example, BICMOS integrated circuits have high performance in terms of speed and current drive capability due to the use of bipolar devices while the use of CMOS devices allows the same circuit to also have high density and low power consumption features.
There have been many problems associated with the integration of bipolar and CMOS devices into a single circuit. It is generally impractical to fabricate CMOS devices in a traditionally bipolar structure and vice versa. As a result, the processes employed for fabricating BICMOS integrated circuits are often extremely complicated. In addition to process complexity, the result of many prior art BICMOS processes is the limited performance of bipolar devices due to sacrifices made to integrate the various technologies. A major reason for having bipolar devices in an integrated circuit is for high performance. Accordingly, the performance of bipolar devices in a BICMOS integrated circuit should not be sacrificed. Finally, many existing BICMOS integrated circuits require extremely large amounts of real estate. This is undesirable.
Accordingly, it would be highly desirable to have a method for fabricating semiconductor structures having MOS and bipolar devices that has process integration flexibility, enhanced scalability characteristics and does not sacrifice performance of the bipolar devices.